At a certain input current above a threshold level, a laser diode changes from a light emitting diode (LED) mode of operation into a laser mode of operation. In the LED operation mode, the laser diode emits incoherent light that diffuses rapidly. The LED operation mode cannot be used for data transmissions over long distances because optical power is lost too quickly. In the laser operation mode, however, the emitted laser light narrows into a coherent beam of light that can be transmitted over long distances with relatively little power loss. Diodes within the same class or family generally have threshold current levels that vary within a relatively narrow range. The threshold current level of a particular laser diode in a given class, however, may vary significantly with temperature. Threshold current levels also may change significantly with the age of a laser diode.
In many applications, the optical power output from a laser diode must be tightly controlled. The high end of laser output power typically is limited by eye safety considerations, laser reliability and receiver saturation levels, whereas the low end of laser output power typically is constrained by the speed performance of the laser source and the link budget. Thus, there typically is only a narrow band of optical power output levels at which a laser may operate to achieve satisfactory safety and performance specifications. However, over time the aging effect may change the delicately set laser power level to extend beyond the prescribed power limits, at which point the laser source must be replaced. To avoid this problem, the optical power output of a laser diode typically is monitored and the input current to the laser diode typically is adjusted in order to achieve a specified output power that satisfies both data transmission and safety requirements. Typically, laser output power is monitored by a photodetector that converts laser optical power into an electrical feedback signal that is transmitted to a feedback control system, which adjusts the laser output power to the specified output power level.
Different approaches have been proposed for monitoring the optical power output from a laser array. In some of these approaches, the optical power output from one or more lasers in the array is monitored. The monitored lasers are used as representative lasers in the array and are not used for data transmission. In another approach, U.S. patent Publication No. 2002/00759A1 describes an arrangement in which every laser diode in a parallel channel laser array is monitored to ensure that every diode is operating at its target level. In this approach, a diffractive optical arrangement in each channel splits off samples of the laser output. The laser output samples are reflected from one or more reflective regions toward a detector that monitors the output power of the samples. A feedback circuit adjusts each laser based on the signals that are generated by the detector. In this approach, the laser array and the diffractive optical arrangement are aligned by alignment pins and held spaced-apart within a transmitter package assembly.